The present invention relates to methods of using an ion exchange membrane to treat fluids.
Ion exchange membranes are used in a variety of applications. Cation exchange membranes, for example, have become increasingly significant, primarily due to their application in electrochemical cells in chlor-alkali production. For example, fluoropolymeric cation exchange membranes, such as NAFION(copyright) (DuPont), have enjoyed widespread development as barriers separating the anode and cathode compartments of electrolytic cells used in the production of chlorine and caustic. The success of fluoropolymeric cation membranes in chlor-alkali production is attributed to the ability of the membranes to allow selective permeation of sodium ions in NaCl solutions while rejecting chloride ions, resulting in cleaner caustic solutions having substantially lower salt content.
In addition to functioning as an ion barrier, ion exchange membranes provide an attractive means for ion removal. The removal of ions is important in many industries, such as the microelectronics industry, where ion contaminants in very small concentrations can adversely affect the performance and fabrication of microchips. For example, the ability to prepare positive and negative photoresists with ultra low levels of metal ion contaminants, without requiring the addition of chemical reagents, is highly desirable in microlithography. Further, the ability to remove anionic and cationic impurities to provide ultra high purity water, for example, is highly desirable in deionized water purification, desalination of brine, and removal of ionic contaminants from pharmaceutical compositions.
While fluoropolymeric cation exchange membranes are widely used as permselective electrochemical barriers in caustic production processes, their solid homogeneous structure and low fluid flux across the membrane make them unsuitable as ion filtration devices. In particular, fluoropolymeric cation exchange membranes, such as NAFION(copyright) (DuPont), generally are considered nonporous and exhibit no appreciable fluid flow rate therethrough, thereby rendering such membranes unsuitable for filtration applications. Further, fluoropolymeric membranes tend to be incompatible with various organic solvents due to swelling and instability. Similarly, other types of ion exchange membranes and ion exchange resins are limited in their applicability in ionic filtration applications due to low fluid flux, organic solvent instability, and low ion exchange capacity.
Further, porous media to which are applied ion exchange resins for the removal of ions have substantial limitations. Such ion exchange media are limited by the characteristics and extent of the ion exchange resin on the surface of the porous media. In particular, the ion exchange reaction is essentially limited to the exposed resin surface area, and, therefore, a large portion of unexposed resin is unused and does not participate in the ion exchange process. Moreover, the generally discrete (rather than continuous or homogeneous) positioning of ion exchange resins on the porous media leads to significant areas of the porous media not participating in the ion exchange process. As a result, such ion exchange media exhibit poor performance (e.g., poor kinetic properties).
Thus, there exists a need for an ion exchange membrane having high ion exchange capacity, stability in a variety of solvents, particularly organic solvents, and a fluid flux suitable for application in filtration. Moreover, there exists a need for materials and methods which enable the removal of trace heavy metal ions from organic and aqueous solvents. Further, there exists a need for materials and methods which enable the removal of trace ionic contaminants from organic solvents, particularly photoresist solvents. The present invention provides such an ion exchange membrane and associated methods. These and other objects of the present invention will be apparent from the detailed description of the preferred embodiments of the invention set forth below.
The present invention provides an ion exchange membrane having a porous polymeric support grafted with an organic moiety having at least one ion exchange group covalently bonded thereto.
The present invention also provides a method for preparing an ion exchange membrane wherein a porous polymeric support is contacted with at least one organic monomer, which is then grafted to the porous polymeric support. At least one ion exchange group is subsequently introduced and covalently bonded to the grafted organic moiety to provide a porous polymeric support grafted with an organic moiety having at least one ion exchange group.
The present invention further provides a method for preparing an ion exchange membrane wherein a porous polymeric support is contacted with at least one organic monomer bearing at least one ion exchange group covalently bonded thereto. The organic monomer is then grafted to the porous polymeric support, thereby providing a porous polymeric support grafted with an organic moiety having at least one ion exchange group covalently bonded thereto.
The present invention provides a method for treating a fluid containing ions wherein ions are removed by contacting the fluid with an ion exchange membrane of the present invention. In addition, the present invention provides a filtration apparatus for removing ions from a fluid, resulting in a filtrate having a lower ion concentration. The filtration apparatus of the present invention is useful for removing trace ion contaminants from organic and aqueous solvents to provide ultra high purity solvents with extremely low concentrations of ionic contaminants. The filtration apparatus has a filter unit which utilizes an ion exchange membrane of the present invention and a means for passing the fluid through the filter unit.